Valeria is playing with her accordion. the length of the accordion a ( t ) a(t) (in cm cm) after she starts playing as a function of time t t (in seconds) can be modeled by a sinusoidal expression of the form a ⋅ cos ⁡ ( b ⋅ t ) + d a⋅cos(b⋅t)+d. at t = 0 t=0, when she starts playing, the accordion is 1 5 cm 15 cm long, which is the shortest it gets. 1 . 5 1.5 seconds later the accordion is at its average length of 2 1 cm 21 cm. find a ( t ) a(t).